Sprinklers are known which can be swung back and forth over such a sector by the reaction force of the water spray impinging on respective faces of a pair of arms that are vertically oscillatable on the barrel, each of these faces forming part of a deflecting head which in a withdrawn position lies above the spray path and in a working position is operatively algined with the nozzle. When the rotating barrel reaches the terminal position of its forward swing, a stop on its mounting deactivates the forward-driving arm by upwardly retracting its deflecting head while operatively positioning the reverse-driving arm; when the barrel returns to the initial or home position of its swing, the positions of its arms are again reversed.
Generally, the deflecting head of the forward-driving arm is biased upwardly by a counterweight and has a reaction face so shaped that the impinging jet, aside from swinging the barrel in the forward direction, tends to move that head downwardly and out of the path of the spray. When this happens, the reaction force is briefly until the counterweight again raises the head into its working position which is defined by an abutment on that arm whereupon the operation is repeated. In this way, the barrel is advanced intermittently to the end of its forward sweep. Reference may be made in this connection to U.S. Pat. No. 3,580,507 showing such a drive mechanism.
In a reciprocating sprinkler as here contemplated, the return swing proceeds continuously rather than intermittently as the reverse-driving arm maintains its working position throughout that swing. Conventionally, the lowering of the deflecting head of this arm into alignment with the nozzle against the force of a restoring spring is arrested by a fixed abutment so that its reaction face cannot escape downwardly from the impact of the jet, contrary to the aforedescribed mode of operation of the forward-driving arm. A drawback of this arrangement resides in the fact that variations in the water pressure may lead to operating irregularities. Thus, if the restoring spring is weak enough to allow even a minimum pressure to return the barrel to its home position, larger pressures may excessively accelerate the barrel as the spray strikes the confronting reaction face with its full force; such an acceleration may damage the swivel mounting of the barrel or dislocate the stops which reverse the positions of the driving arms at the end of the return swing.